The following invention is in the field of acoustic analysis, spatial sound recording, microphone array signal processing, and spatial filtering. Some embodiments of the present invention relate to a method that can be used to determine the filter coefficients of a diffuse sound filter, i.e., a filter for extracting diffuse sound (reverberant sound) from the recordings with a microphone array. Some embodiments relate to a corresponding computer program. Some embodiments relate to an apparatus that can be used to determine the filter coefficients of a diffuse sound filter.
Sound acquisition with microphone arrays in reverberant environments typically aims at capturing the direct sound of the sound sources while attenuating noise and reverberation. For many applications it would be beneficial if we were able to extract also the reverberant sound while suppressing the direct sound and noise. For instance in spatial sound reproduction [Pulkki2007, Thiergart2013, Kowalczyk2013], the reverberation present at the recording side needs to be reproduced at the reproduction side to recreate the desired spatial impression. Moreover, given an estimate of the reverberant sound, we can compute parameters such as the signal-to-reverberation ratio or reverberant sound power, which represent crucial information for various other applications.
While the estimation of direct sound components (e.g., using source separation, dereverberation, or noise reduction) is well addressed in literature, only few approaches exist for extracting reverberant sound. Usually, reverberation is modeled as a (time-varying) diffuse sound field. To extract the diffuse sound, single-channel filters have been used recently (e.g., in [Pulkki2007, Thiergart2013]), which yield poor performance when multiple sources are active or for transient-like signals. A better performance can be achieved with multi-channel filters (e.g., [Kowalczyk2013, Thiergart2013b]). Unfortunately, currently existing multi-channel filters are not optimal and do not yield a suitable directivity pattern for capturing diffuse sound.
It would therefore be desirable to provide a diffuse sound filter having improved performance in terms of diffuse sound extraction and/or direct sound suppression. It may also be desirable that the diffuse sound filter has a directional response that is highly omnidirectional, with the exception of directions of arrival of direct sound components. A highly omnidirectional directional response is desired since the diffuse sound arrives from all directions at the microphone array.